The present invention relates to a positioning device which utilizes signals from positioning satellites, a method of controlling a positioning device, and a recording medium.
A positioning system has been used which locates the present position of a GPS receiver utilizing a global positioning system (GPS) (satellite navigation system) which is an example of a satellite positioning system (SPS).
The GPS receiver receives signals from three or more GPS satellites, and calculates the distance between each GPS satellite and the GPS receiver (hereinafter called “pseudo-range”) from the difference between the time at which the signal is transmitted from each GPS satellite and the time at which the signal reaches the GPS receiver (hereinafter called “delay time”), for example. The GPS receiver calculates (locates) the present position using the pseudo-range and satellite orbital information of each GPS satellite contained in the signal received from each GPS satellite.
However, when the signal from the GPS satellite reaches the GPS receiver after being reflected by a building or the like, or the signal strength is weak, or the dilution of precision (DOP) of the GPS satellite in the sky is low, the located position may differ from the true position to a large extent, whereby the accuracy of the located position may deteriorate.
A technology has been proposed which calculates the present expected position (hereinafter called “expected position”) from the velocity vector and the elapsed time based on the preceding located position, and averages the expected position and the present located position (e.g. JP-A-8-68651 (e.g. FIG. 5)).
However, the GPS satellite moves in the satellite orbit even when the GPS receiver is stationary, and the reception state of the satellite signal changes from moment to moment. Therefore, the velocity indicated by the velocity vector does not necessarily become zero.
According to the above technology, when the elapsed time is 10 seconds (s), the expected position differs from the preceding position at a distance corresponding to 10 seconds even when the GPS receiver is stationary. As a result, the accuracy of the position obtained by averaging deteriorates, whereby the output position may differ from the true position.
According to the above technology, when the GPS receiver is stationary, the expected position cumulatively differs from the preceding position with the passage of time from the preceding positioning, whereby the output position differs from the true position.
Moreover, when the preceding located position differs from the true position and lacks reliability, the reliability of the expected position and the average position also deteriorate.
When a GPS receiver carried by a pedestrian moves at a low speed, the present located position may differ from the preceding located position to a large extent depending on the velocity indicated by the velocity vector, although the actual present position is near the preceding located position.